An underground ammunition depot in Libya's desert - before (left) and after it was destroyed. Photograph: Richard Norton-Taylor

British and US aircraft have destroyed ammunition dumps deep in the desert 500 miles south of the Libyan coast in one of their longest joint bombing runs.

As B1 long-range bombers took off from their base in the US, in the early hours of Monday two Tornados flew from RAF Marham in Norfolk. The US bombers fired 500lb joint direct attack munitions, the Tornados, refuelled en route, fired Storm Shadow air-launched cruise missiles (at £800,000 a piece). In total 40 underground munitions bunkers near Sabha were destroyed.

The bombing was justified on the grounds that it indirectly protected civilians, denying Gaddafi forces ammunition that would have become increasingly important as fighting continues in Mistrata, and the town of Zintan to the west.

Liam Fox, the UK defence secretary, said: "With targeted strikes like this we are hitting Gaddafi's forces where it hurts, limiting their supply lines and in turn reducing their capability to kill their own civilians."

In eleven days since the start of the military operation RAF Tornados have fired Brimstone "precision-guided" missiles, Storm Shadow "stand-off" cruise missiles, and Paveway IV bombs. HMS Triumph, which has now left the Mediterranean replaced by her sister boat, HMS Turbulent, has fired seven Tomahawks.

Unlike the US, the MoD does not release the number of weapons fired by British Misrata aircraft. The US defence department says American forces have fired almost 200 Tomahawk missiles from submarines and surface ships and dropped 455 bombs.

Ten RAF Eurofighter-Typhoons have been involved in the military operations – though by Wednesday they had not yet fired any weapons – eight Tornados and 24 transport aircraft, including two large chartered Antonov transport planes, with 417 tonnes of supplies deployed to UK forces in the area by air and RAF tanker aircraft which had been involved in 130 hours of air-to-air refuelling.